This invention relates to composite polyurethane foams and methods of manufacture thereof. In particular, this invention relates to a layered polyurethane foamed composite.
Composite, i.e., multiple-layer foamed polyurethane materials are known, and are often manufactured in order to obtain a combination of desirable properties. Such composites have primarily been manufactured by laminating the individual foam layers together, generally by using adhesives. These methods require additional manufacturing steps, and may result in materials with degraded, rather than improved properties.
A composite, dual layer polyurethane foam is disclosed in U.S. Pat. No. 5,859,081 to Duffy, wherein the composite is manufactured by casting an uncured (wet) polyurethane foam composition onto a cured (dry) polyurethane foam. It is admitted in Duffy that the dual layer foam has a xe2x80x9cfine and uniform interface between the two layersxe2x80x9d (col. 2, lines 59-60). However, such an interface line can interfere with air flow through the multilayer foam structure, reducing the breathability, porosity and water vapor transmission that are important for many applications. In addition, this interface can produce a location for delamination of the layers because it""s strength depends on the adhesion of the second cast layer to the first.
It has also been found by the inventors that casting uncured wet polyurethane foam compositions onto a cured (dry) polyurethane open celled foam results in large voids and bubbles in the second layer, due to expansion of the gas in the cells of the first layer during the curing process. In addition to the cosmetic deficiencies of these voids and bubbles the physical properties can also be negatively affected during production and/or use.
Thus, there is a need for a product and a process to make a multilayer foam structure with no structural interface line between the layers.
A method for the manufacture of multiple layer polyurethane foams, which comprises:
casting an uncured (wet) polyurethane foam onto a cast, uncured (wet) polyurethane foam; and
curing the resulting layers of foam such that the final product possesses no discernible structural interface line between the layers when examined at a magnification of 50xc3x97 using a scanning electron microscope (SEM).
If the different layers have different colors then visible microscopy can be used to visualize the border region between layers. Multilayered foams of the present invention show evidence of polymer diffusion between the layers but the cells of the foam are distributed uniformly from one surface to the other without regard to the transition zone between the layers. Such multilayered foams cannot be delaminated, in that the tear strength of one of the layers is exceeded before any evidence of delamination is seen.